Electrolysis

Equations Worksheet

Question 1

Answer

Question 2A

Answer

Question 2B

Answer

Questions

Question 1(2008)

During the electrolysis of molten lead bromide, which of the following takes place ?

1. Bromine is released at the cathode
2. Lead is deposited at the anode
3. Bromine ions gain electrons
4. Lead is deposited at the cathode

Answer

Lead is deposited at the cathode

Reaction at cathode : Pb²⁺ + 2e^- ⟶ Pb

Question 2.1(2008)

Here is an electrode reaction:

Cu ⟶ Cu²⁺ + 2e^-. [i.e. Cu – 2e^- ⟶ Cu²⁺]

At which electrode (anode or cathode) would such a reaction take place ? Is this an example of oxidation or reduction ?

Answer

Cu ⟶ Cu²⁺ + 2e^-

This reaction takes place at anode.

This is an example of oxidation.

Question 2.2(2008)

A soln. contains Mg²⁺ ions, Fe²⁺ ions and Cu²⁺ ions. On passing an electric current through this soln. which ions will be the first to be discharged at the cathode? Write the equation for the cathode reaction.

Answer

Cu²⁺ (Copper ions will get discharged at cathode)

Reaction at Cathode:
Cu²⁺ + 2e^- ⟶ Cu

Question 1(2009)

(A) Sulphur

(B) Silver chloride

(C) Hydrogen chloride

(D) Copper [II] sulphate

(E) Graphite

State which from A to E fits the description — 'A pink metal is deposited at the cathode during the electrolysis of the soln. of this salt.

Answer

(D) Copper [II] sulphate

At cathode : Cu²⁺ + 2e^- ⟶ Cu

Question 2(2009)

Select the correct answer — The aqueous solution of the compound which contains both ions and molecules is:

1. Sulphuric acid
2. Hydrochloric acid
3. Nitric acid
4. Acetic acid

Answer

Acetic acid
Reason — Acetic acid is a weak electrolyte and particles in a weak electrolyte are ions and unionized molecules.

Question 3(2009)

Correct the following statements — Lead bromide conducts electricity.

Answer

Molten lead bromide conducts electricity.
Reason — Solid lead bromide is a non-conductor of electricity since it's ions are not free but held together by an electrostatic force of attraction. In molten state the ions become free and hence conduct electricity.

Question 4(2009)

A metal article is to be electroplated with silver. The electrolyte selected is sodium argentocyanide.

(i) What kind of salt is sodium argentocyanide.

(ii) Why is it preferred to silver nitrate as an electrolyte?

(iii) State one condition to ensure that the deposit is smooth, firm and long lasting.

(iv) Write the reaction taking place at the cathode.

(v) Write the reaction taking place at the anode.

Answer

(i) It is a Complex Salt.

(ii) Migration of Ag¹⁺ ions from above complex salt soln. is slow compared to that from silver nitrate. Hence, an even deposition of metal silver is obtained on the article. Therefore, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) A low current for a longer time should be used to ensure that the deposit is smooth, firm and long lasting.

(iv) Ag¹⁺ + 1e^- ⟶ Ag [deposited]

(v) Ag - 1e^- ⟶ Ag¹⁺ [Anode diminishes in mass]

Question 5(2009)

Aqueous solution of nickel sulphate contains Ni²⁺ and SO₄^2- ions

(i) Which ion moves towards the cathode?

(ii) What is the product at the anode?

Answer

(i) Nickel ion moves towards the cathode.

(ii) Product at anode - Nil [Ni²⁺ are formed]

Question 1(2010)

Select the correct answer — A compound which during electrolysis in it's molten state liberates a reddish brown gas at the anode :

1. Sodium chloride
2. Copper (II) oxide
3. Copper (II) sulphate
4. Lead (II) bromide

Answer

Lead (II) bromide
Reason — During electrolysis of Lead [II] bromide, Br^-1 ions are discharged at the anode. Reddish brown fumes are due to bromine vapours.

Question 2(2010)

During electroplating of an article with nickel —

(i) Name — (a) The electrolyte (b) The cathode (c) The anode

(ii) Give the reaction of the electrolysis at (a) The cathode (b) The anode

Answer

(i) (a) The electrolyte — Aq. soln. of nickel sulphate (b) The cathode — Cleaned article to be electroplated (c) The anode — Plate or block of nickel metal

(ii) the reaction of the electrolysis at:
(a) At Cathode: Ni²⁺ + 2e^– ⟶ Ni
(b) At Anode : Ni – 2e^– ⟶ Ni²⁺

Question 3(2010)

A, B and C are three electrolytic cells, connected in different circuits. Cell 'A' contains NaCl soln., and the bulb in the circuit glows brightly, when the circuit is completed. Cell 'B' contains acetic acid and the bulb glows dimly. Cell 'C' contains sugar soln., and the bulb does not glow. Give reasons for each observation.

Answer

When the switch is 'On' the electrolyte starts dissociating. Current remaining the same, the glow of the bulb indicates whether the electrolyte is a strong, weak or a non-electrolyte.

As NaCl is a strong electrolyte (i.e., cell A), therefore it allows large amount of electricity to flow through it. Hence, the bulb glows brightly.

Acetic acid is a weak electrolyte (i.e., cell B) and it allows a small amount of electricity to flow through them hence, the bulb glows dimly.

Sugar soln. is a non-electrolyte (i.e., cell C), therefore it does not conduct electricity and do not undergo chemical decomposition due to the flow of current through it. Hence, the bulb does not glow.

Question 1(2011)

Give reasons – The electrolysis of acidulated water is considered to be an example of catalysis.

Answer

Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H₂SO₄ which dissociates as: H⁺¹ and SO₄^2- and help in dissociating water into H⁺¹ and OH^- ions, water being a polar solvent. Hence, electrolysis of acidulated water is considered to be an example of catalysis.

Question 2(2011)

During electrolysis of CuSO₄ solution using platinum [cathode] and carbon [anode]:

(i) State what you observe at the cathode and anode.

(ii) State the change noticed in the electrolyte

(iii) Write the reactions at the cathode and anode.

Answer

(i) At cathode : Cu²⁺ ions and H¹⁺ ions migrate to the cathode. Cu²⁺ ions are discharged in preference to H¹⁺.

At anode : SO₄^2- and OH^1- ions migrate to the anode but OH^1- ions are discharged since they are lower in the electrochemical series.

(ii) The blue colour of CuSO₄ solution fades since the blue Cu²⁺ ions which are discharged at the cathode are not replaced at the anode.

(iii) Cathode : Cu²⁺ + 2e^- ⟶ Cu

Anode: OH^1- - 1e^- ⟶ OH x 2

[2OH ⟶ H₂O + [O]]

4OH ⟶ 2H₂O + O₂

Question 3(2011)

Differentiate between electrical conductivity of – copper sulphate solution and of copper metal.

Answer

Question 1(2012)

Rewrite the correct statement with the missing word/s: Cations migrate during electrolysis.

Answer

Cation migrates to the cathode during electrolysis.

Question 2(2012)

Identify the weak electrolyte from the following:

1. Sodium Chloride solution
2. Dilute Hydrochloric acid
3. Dilute Sulphuric acid
4. Aqueous acetic acid.

Answer

Aqueous acetic acid

Question 3(2012)

Match the following in column A with the correct answer from the choices given in column B.

Answer

Question 4(2012)

Give reasons: An aqueous solution of sodium chloride conducts electricity.

Answer

The ions of sodium chloride are not free but held together by strong electrostatic force of attraction. However in aqueous solution of sodium chloride the ions break lose and move freely. Hence, an aqueous solution of sodium chloride conducts electricity.

Question 5(2012)

Select the correct answer from the list in brackets:

(i) An aqueous electrolyte consists of the ions mentioned in the list. The ion which could be discharged most readily during electrolysis. [Fe²⁺, Cu²⁺, H⁺].

(ii) The metallic electrode which does not take part in an electrolytic reaction. [Cu, Ag, Pt, Ni].

(iii) The ion which is discharged at the anode during the electrolysis of copper sulphate solution using copper electrodes as anode and cathode. [Cu²⁺, OH^1-, SO₄^2-, H⁺].

(iv) When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily. [Na¹⁺, OH^1-, H¹⁺, Cl^1-].

(v) During silver plating of an article using potassium argentocyanide as an electrolyte, the anode material should be [Cu, Ag, Pt, Fe].

Answer

(i) An aqueous electrolyte consists of the ions mentioned in the list. The ion which could be discharged most readily during electrolysis. — Cu²⁺.

(ii) The metallic electrode which does not take part in an electrolytic reaction. — Pt.

(iii) The ion which is discharged at the anode during the electrolysis of copper sulphate solution using copper electrodes as anode and cathode. — Cu²⁺.

(iv) When dilute sodium chloride is electrolysed using graphite electrodes, the cation which is discharged at the cathode most readily. — H¹⁺

(v) During silver plating of an article using potassium argentocyanide as an electrolyte, the anode material should be — Ag.

Question 1(2013)

State one appropriate observation for : Electricity is passed through molten lead bromide.

Answer

Solid lead bromide is a non conductor of electric current since it's ions are not free and held together by strong electrostatic force of attraction. The ions become free in molten state. Hence, electricity is passed through molten lead bromide.

Question 2(2013)

State which of these will act as a non-electrolyte ?

1. Liquid carbon tetrachloride
2. Acetic acid
3. Sodium hydroxide aqueous solution acid.
4. Potassium chloride aqueous solution.

Answer

Liquid carbon tetrachloride is a non-electrolyte

Question 3(2013)

Copper sulphate soln. is electrolysed using copper electrodes as seen in diagram.

(i) Which electrode to your left or right is known as the oxidising electrode and why?

(ii) Write the equation representing the reaction that oc­curs.

(iii) State two appropriate observations for the above elec­trolysis reaction.

Answer

(i) The electrode connected to the positive terminal of the battery (i.e., electrode on the left) is the oxidizing electrode as anions donate excess electrons to the anode and are oxidized to neutral atoms.

(ii) Cu – 2e^– ⟶ Cu²⁺

(iii) Copper anode shows a loss in mass. The blue colour of aq. copper sulphate soln. remains unchanged.

Question 4(2013)

Name: A gas which in the liquid state does not conduct electricity but conducts when dissolved in water.

Answer

Hydrogen chloride gas

Question 1(2014)

The observation seen when fused lead bromide is electrolysed is:

1. a silver grey deposit at anode and a reddish brown deposit at cathode.
2. a silver grey deposit at cathode and a reddish brown deposit at anode.
3. a silver grey deposit at cathode and reddish brown fumes at anode.
4. silver grey fumes at anode and reddish brown fumes at cathode.

Answer

a silver grey deposit at cathode and reddish brown fumes at anode.
Reason — silver grey colour is due to lead metal deposited at cathode and reddish brown fumes at anode is due to bromine vapours.

Question 2(2014)

During electroplating an article with silver, the electrolyte used is:

1. silver nitrate solution
2. silver cyanide solution
3. sodium argentocyanide solution
4. nickel sulphate solution

Answer

sodium argentocyanide solution

Question 3(2014)

Give one word or phrase for: Electrolytic deposition of a superior metal on a base metal.

Answer

Electroplating

Question 4(2014)

State your observation: At the cathode when acidified aq. CuSO₄ soln. is electrolysed with copper electrodes.

Answer

Copper, a brownish pink metal is deposited at the cathode when acidified aq. CuSO₄ soln. is electrolysed with copper electrodes

Question 5(2014)

State which electrode: anode or cathode is the oxidizing electrode. Give a reason for the same.

Answer

Anode is the oxidizing electrode.
Reason — The anions donate the excess electrons to the anode and are oxidized to neutral atoms. Hence, the anode is the oxidizing electrode by which the electrons leave the electrolyte.

Question 6(2014)

Name the kind of particles present in:

(i) Sodium hydroxide soln.

(ii) Carbonic acid.

(iii) Sugar soln.

Answer

(i) Strong electrolyte — Sodium (Na⁺) ions and hydroxide (OH^-) ions.

(ii) Weak electrolyte — Ions (H⁺, HCO₃, CO₃^2- ) and molecules (H₂CO₃)

(iii) Non-electrolyte — Molecules of sugar soln.

Question 7(2014)

M₂O is the oxide of a metal 'M' which is the above hydrogen in the activity series. M₂O when dissolved in water forms the corresponding hydroxide which is a good conductor of electricity.

(i) State the reaction taking place at the cathode.

(ii) Name the product at the anode.

Answer

Reaction at cathode : M⁺ + e^- ⟶ M

Product at anode : Oxygen

Question 1(2015)

State one observation for. 'At the anode when aqueous copper sulphate solution is electrolysed using copper electrodes'.

Answer

Copper anode shows a loss in mass.

Question 2(2015)

Give scientific reasons :

(i) During electrolysis of molten lead bromide, graphite anode is preferred to other electrodes.

(ii) Electrical conductivity of acetic acid is less in comparison to that of dil. H₂SO₄ – at a given concentration.

(iii) Electrolysis of molten lead bromide is considered to be a redox reaction.

Answer

(i) As graphite is unaffected by the reactive bromine vapours released at the anode hence, a graphite anode is preferred during the electrolysis of molten lead bromide.

(ii) Dil. H₂SO₄ is a strong electrolyte and acetic acid is a weak electrolyte. Therefore, dil. H₂SO₄ allows large amount of electricity to flow through them and are good conductors of electricity whereas acetic acid allows small amount of electricity to flow through them and are poor conductors of electricity. Hence, electrical conductivity of acetic acid is less in comparison to that of dil. H₂SO₄ – at a given concentration.

(iii) Electrolysis of molten lead bromide involves oxidation and reduction reactions and hence is a redox reaction.

Reduction reaction at cathode:
Pb²⁺ + 2e^- ⟶ Pb

Oxidation reaction at the anode:
Br^1- - 1e^- ⟶ Br
Br + Br ⟶ Br₂

Question 3(2015)

Differentiate between the terms strong electrolyte and weak electrolyte. (stating any two differences)

Answer

Question 4(2015)

During purification of copper – State:

(i) The anode used

(ii) The electrolyte used.

Write the equation taking place at the anode in the above.

Answer

(i) Impure block of copper

(ii) Aq. copper sulphate solution [acidified]

Equation at the anode: Cu – 2e^- ⟶ Cu²⁺

Question 1(2016)

The particles present in strong electrolytes are:

Select the correct answer from A, B, C and D:

A: only molecules
B: mainly ions
C: ions and molecules
D: only atoms

Answer

mainly ions

Question 2(2016)

Write equations for the reactions taking place at the two electrodes during the electrolysis of:

(i) Acidified CuSO₄ solution with copper electrodes.

(ii) Molten PbBr₂ with inert electrodes. (mention clearly the name of the electrode in each case)

Answer

(i) Electrode — Cathode : Copper, Anode : Copper

Dissociation of aq. CuSO₄ :

CuSO₄ ⇌ Cu²⁺ + SO₄^2-

H₂O ⇌ H¹⁺ + OH^1-

H₂SO₄ ⇌ 2H¹⁺ + SO₄^2-

Reaction at cathode [pure thin sheet of Cu]:
Cu²⁺ + 2e^- ⟶ Cu [Cu deposited on thin sheet]

Reaction at anode [impure block of active Cu]:
Cu - 2e^- ⟶ Cu²⁺ [product nil - Cu²⁺ ions]

(ii) Electrode — Cathode : Iron or Graphite, Anode : Graphite

Dissociation of molten PbBr₂ : PbBr₂ ⇌ Pb²⁺ (cathode) + 2Br^1- (anode)

Reaction at cathode:
Pb²⁺ + 2e^- ⟶ Pb [product lead metal]

Reaction at anode:
Br^1- - 1e^-⟶ Br
Br + Br ⟶ Br₂

Question 3(2016)

Name:

(i) the product formed at the anode – during electrolysis of acidified water using platinum electrodes.

(ii) the metallic ions that should be present – in the electrolyte when an article made of copper is to be electroplated with silver.

Answer

(i) Oxygen

(ii) Silver ions

Question 4(2016)

Give reasons why:

(i) Sodium Chloride will conduct electricity only in fused or aq. soln. state.

(ii) In the electroplating of an article with silver, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) Although copper is a good conductor of electricity, it is a non-electrolyte.

Answer

(i) The ions Na⁺ and Cl^– are not free but held together by strong electrostatic force of attraction. In fused or molten state the ions break free and move. Hence, NaCl will conduct electricity only in fused state or aq. soln. state.

(ii) Migration of Ag¹⁺ ions from sodium argentocyanide solution is slow compared to that from silver nitrate. Hence, an even deposition of metal silver is obtained on the article. Therefore, the electrolyte sodium argentocyanide soln. is preferred over silver nitrate solution.

(iii) Copper does not undergo chemical decomposition due to flow of electric current through it. Hence, copper is a good conductor of electricity but it is a non-electrolyte.

Question 1(2017)

Identify the substance underlined, in each of the following —

(i) The electrolyte used for electroplating an article with silver.

(ii) The particles present in a liquid such as kerosene, that is a non-electrolyte.

Answer

(i) Sodium argentocyanide

(ii) Molecules only

Question 2(2017)

State the observations at the anode & at the cathode during the electrolysis of –

(i) Fused lead bromide using graphite electrodes.

(ii) Copper sulphate solution using copper electrodes.

Answer

(i) The observations at the anode & at the cathode are:
At cathode — silvery grey deposit of lead metal.
At anode — reddish brown fumes of bromine vapours.

(ii) The observations at the anode & at the cathode are:
At cathode — Brownish pink copper metal is deposited at cathode during electrolysis of copper sulphate.
At anode — Copper ions are formed. Copper anode diminishes in mass.

Blue colour of CuSO₄ remains unchanged.

Question 3(2017)

Select the ion in each case, that would get selectively discharged from the aqueous mixture of the ions listed below:

(i) SO₄^2-, NO₃^- and OH^- ;

(ii) Pb²⁺, Ag⁺ and Cu²⁺.

Answer

(i) OH^- ion will get discharged in preference to SO₄^2- or NO₃^- ions.

(ii) Ag⁺ ions will get discharged in preference to Pb²⁺ or Cu²⁺ ions.

Question 1(2018)

Choose the correct answer from the options given : The electrolysis of acidified water is an example of :

1. Reduction
2. Oxidation
3. Redox reaction
4. Synthesis

Answer

Redox reaction
Reason — Reaction at cathode: Hydrogen ions, H⁺ being the only positively charged ions migrate to the cathode. Since, the cathode is a reservoir of electrons , H⁺ ions gain an electron and become a neutral hydrogen atom. Therefore reduction takes place at cathode.

H⁺ + e^- ⟶ H

H + H ⟶ H₂

Reaction at anode : SO₄^2- and OH^- ions both migrate to the anode. OH^-being lower in electrochemical series is discharged preferentially.

OH^- ions loses an electron to the anode and becomes neutral OH. The combination of OH forms water with the liberation of oxygen, given off at anode.

OH^- ⟶ OH + e^-

OH + OH ⟶ H₂O + O

O + O ⟶ O₂

Therefore, oxidation takes place at anode. Hence, electrolysis of acidified water is an example of redox reaction

Question 2(2018)

State one relevant observation : At the anode when molten lead bromide is electrolysed using graphite electrodes

Answer

At the anode, brown fumes of bromine vapours are observed.

Question 3(2018)

Give a reason for : Conductivity of dilute hydrochloric acid is greater than that of acetic acid.

Answer

Dil. H₂SO₄ is a strong electrolyte and acetic acid is a weak electrolyte. Therefore, dil. H₂SO₄ allows large amount of electricity to flow through it and is a good conductor of electricity whereas acetic acid allows small amount of electricity to flow through it and is a poor conductor of electricity. Hence, electrical conductivity of dilute hydrochloric acid is greater than that of acetic acid.

Question 4(2018)

Name the gas that is produced at the anode during the electrolysis of acidified water.

Answer

Oxygen gas

Question 5(2018)

Fill in the blank with the correct choice given in the bracket - Electrolysis of aqueous sodium chloride solution will form ............... at the cathode. [hydrogen gas / sodium metal]

Answer

Electrolysis of aqueous sodium chloride solution will form hydrogen gas at the cathode.

Question 6(2018)

Copy and complete the following table which refers to the conversion of ions to neutral particles.

Answer

Question 7(2018)

For the electro-refining of copper —

(i) State what the cathode is made up of.

(ii) Write the reaction that takes place at the anode.

Answer

(i) Pure thin sheet of copper.

(ii) Cu - 2e^- ⟶ Cu²⁺

Question 1(2019)

Choose the correct answer from the options A, B, C, D given. An electrolyte which completely dissociates into ions is

1. Alcohol
2. Carbonic acid
3. Sucrose
4. Sodium hydroxide

Answer

Sodium hydroxide
Reason — Sodium hydroxide is a strong electrolyte hence, it completely dissociates into ions.

Question 2(2019)

State one observation for the following : Copper sulphate solution is electrolysed using copper electrodes.

Answer

A brownish pink deposit of copper metal is seen at the cathode when copper solution is electrolyzed using copper electrodes. The blue colour of Copper Sulphate solution does not face and Copper anode diminishes in mass.

Question 3(2019)

Arrange the following according to the instruction given in the bracket : Mg²⁺, Cu²⁺, Na¹⁺, H¹⁺ [In the order of preferential discharge at the cathode. ]

Answer

Cu²⁺, H¹⁺, Mg²⁺, Na¹⁺

Lower the position of the ion, greater the tendency to be liberated at the cathode (or respective electrode).

Question 4(2019)

Name the particles present in :

(i) Strong electrolyte

(ii) Non-electrolyte

(iii) Weak electrolyte

Answer

(i) Ions only

(ii) Molecules only

(iii) Ions and Molecules

Question 1(2020)

Choose the correct answer from the options given : The inert electrode used in the electrolysis of acidified water is :

1. Nickle
2. Platinum
3. Copper
4. Silver

Answer

Platinum

Question 2(2020)

State one relevant reason for : Graphite anode is preferred to platinum in the electrolysis of molten lead bromide.

Answer

As graphite is unaffected by the reactive bromine vapours released at the anode hence, a graphite anode is preferred to platinum in the electrolysis of molten lead bromide.

Question 3(2020)

Give one word or a phrase for the statement : Electrode used as cathode in electrorefining of impure copper.

Answer

Pure thin sheet of copper is used as cathode.

Question 4(2020)

Fill in the blanks from the choices give in bracket : An alkali which completely dissociates into ions is ............... [ammonium hydroxide, calcium hydroxide, lithium hydroxide]

Answer

An alkali which completely dissociates into ions is lithium hydroxide

Question 5(2020)

Choose the correct word which refers to the process of electrolysis from A to E, to match the description (i) to (iv): A: Oxidation, B: Cathode, C: Anode , D: An electrolyte , E : Reduction

(i) Conducts electricity in aq. or in molten state.

(ii) Loss of electrons takes place at anode.

(iii) A reducing electrode

(iv) Electrode connected to the positive end or terminal of the battery.

Answer

(i) Conducts electricity in aqueous or in molten state — D: An electrolyte

(ii) Loss of electrons takes place at anode — A: oxidation

(iii) A reducing electrode — B: Cathode

(iv) Electrode connected to positive end or terminal of the battery. — C: Anode

Question 6(2020)

Differentiate between the following pairs based on the information given in the bracket. Conductor and electrolyte [conducting particles]

Answer

Question 7(2020)

Identify the underlined substance: The electrode that increases in mass during the electro-refining of silver.

Answer

Cathode
Reason — Pure copper is deposited on the thin sheet of pure copper placed at the cathode.

Question 8(2020)

An aqueous solution of nickle [II] sulphate was electrolyzed using nickle electrodes. Observe the diagram and answer the questions that follow:

(i) What do you observe at the cathode and anode respectively.

(ii) Name the cation that remains as a spectator ion in the solution.

(iii) Which equation for the reaction at the anode is correct.

1. Ni ⟶ Ni²⁺ + 2e^-
2. Ni + 2e^- ⟶ Ni²⁺
3. Ni²⁺ ⟶ Ni + 2e^-
4. Ni²⁺ + 2e^- ⟶ Ni

Answer

(i) The observations at cathode and anode are:
At cathode: Ni²⁺ ions move towards cathode and are discharged as neutral Ni atoms by gain of electrons. Thus, Ni is deposited at cathode.
At anode: Diminishes in mass. Ni²⁺ ions are formed.

(ii) H⁺ ions.

(iii) Ni ⟶ Ni²⁺ + 2e^-

Additional Questions

Question 1

Define :

(i) Electrolysis

(ii) Electrodes

(iii) Ions

(iv) Electrolytic dissociation.

Answer

(i) Electrolysis — It is the decomposition of a chemical compound [electrolyte] in the aqueous or fused [molten] state by the passage of direct electric current resulting in discharge of ions as neutral atoms at the respective electrodes.

(ii) Electrodes — They allow the electric current to enter or leave the electrolytic solution. The electrodes are two in number and are made of metal or carbon.

(iii) Ions — They are atoms which carry a positive or negative charge and become free and mobile when an electric current is passed through an aqueous solution of a chemical compound.

(iv) Electrolytic dissociation — The process due to which an ionic compound in the fused [molten] or in aqueous solution state dissociates into ions by passage of electric current through it is called electrolytic dissociation.

Question 2

Differentiate between

(i) Electrolytes and Non-electrolytes

(ii) Strong and Weak electrolytes

(iii) Anode and Cathode

(iv) Electrolytic dissociation and ionization with suitable examples.

Answer

(i) Differences between Electrolytes and Non-electrolytes are:

(ii) Differences between Strong and Weak electrolytes are:

(iii) Differences between Anode and Cathode are:

(iv) Differences between Electrolytic dissociation and ionization are:

Question 3

Compare the flow of electricity through a nickel wire and nickel sulphate solution.

Answer

Question 4

Name three organic compounds and one neutral liquid which are non-electrolytes.

Answer

Organic compounds which are non-electrolyte — sucrose, glucose, sugar solution

Neutral liquid which are non-electrolytes — liquid carbon tetrachloride

Question 5

State which of the following solutions are weak electrolytes — dil. HCI; carbonic acid; NH₄OH; dil. H₂SO₄; AgNO₃; Na₂CO₃; PbBr₂; KOH; HI; oxalic acid, NaHCO₃; sodium acetate; Na₂SO₄; NaOH.

Answer

Weak Electrolytes — Carbonic acid, NH₄OH, Na₂CO₃ oxalic acid.

Question 6

State which of the following solutions contain:

(i) molecules only

(ii) ions only

(iii) both molecules and ions — CS₂; CH₃COOH ; NH₄OH ; NaOH ; dil. HNO₃ ; Na₂CO₃ ; CuCl₂ ; oxalic acid ; pure H₂O, kerosene ; HI.

Answer

(i) molecules only — pure H₂O, kerosene, CS₂

(ii) ions only — dil. HNO₃, NaOH, CuCl₂, HI

(iii) both molecules and ions — CH₃COOH; NH₄OH; Na₂CO₃; oxalic acid

Question 7

State giving reasons, in which state or medium does (i) NaCl (ii) HCl gas (iii) NH₃ gas conduct electricity.

Answer

(i) NaCl will conduct electricity only in fused state or aq. soln. state. This is because the ions Na⁺ and Cl^– are not free but held together by strong electrostatic force of attraction. Hence, it is a bad conductor of electricity in solid state.

(ii) HCl conducts electricity only when it is dissolved in water. When HCl (polar solvent) is added to water, the slightly negatively charged oxygen atom of water exerts an electrostatic pull on positively charged hydrogen ion of HCl. Thus, H⁺ ions combine with the water, forming hydronium ion and the residual chloride ions remain in water and these ions help in conduction of electricity.

(iii) NH₃ gas conducts electricity only when it is dissolved in water. When NH₃ gas is dissolved in water it forms NH₄OH. As NH₄OH is a weak electrolyte, so it ionizes to a small extent to give (NH₄⁺ and OH^-) ions and these ions help in conduction of electricity.

Question 8

State on what basis are acids, bases and salts classified as strong or weak electrolytes.

Answer

Acids, bases and salts that almost completely dissociate in fused or in aqueous solution state and contain mainly ions only are classified as strong electrolytes.

Acids, bases and salts that partially dissociate in fused or in aqueous solution state and ions as well as unionised molecules are classified as weak electrolytes.

Question 9

Explain the terms :

(i) metal activity or electro chemical series

(ii) selective discharge of ions.

Answer

(i) Depending on the ease with which the metals lose their electrons and form ions, they are arranged in a series known as metal activity series or electro chemical series.
The arrangement is so done that the elements that ionize most readily [discharged with great difficulty] are placed at the top of the series and other elements in the descending order.

(ii) The preferential discharge of ions present in an electrolyte at the respective electrodes is known as selective discharge of ions.

Question 10

From the ions –

(i) SO₄^2– and OH^1–

(ii) Cu²⁺ and H¹⁺

(iii) Ag¹⁺ and H¹⁺

state giving reasons which ion is discharged at the respective electrode in each case.

Answer

(i) OH^1- ions are discharged since they are lower in electrochemical series.

(ii) Cu²⁺ ions are discharged in preference to H¹⁺, as Cu is lower in the electro chemical series than H

(iii) Ag¹⁺ will be discharged because Ag is lower in the electro chemical series than H.

Question 11

With reference to nature of electrodes — name three inert and three active electrodes.

Answer

Three Inert electrodes:

1. Iron
2. Graphite
3. Platinum

Three Active electrodes:

1. Copper
2. Nickel
3. Silver

Question 12

State the reason for difference in product formed at the anode during electrolysis of aq. CuSO₄ using :

(i) active electrode — copper anode

(ii) inert electrode — platinum anode.

Answer

(i) Electrolysis of aq. CuSO₄ using copper anode — Product at anode is nil. [Copper ions are formed]

Reaction at anode: Cu - 2e^- ⟶ Cu²⁺

SO₄^2- and OH^- ions migrate to anode but neither of them are discharged due to the nature of anode, (copper loses electrons more easily than SO₄^2- and OH^-) because copper anode itself ionises to give Cu²⁺ ions.

(ii) Electrolysis of aq. CuSO₄ using inert platinum anode — Product at anode is oxygen gas.

SO₄^2- and OH^- ions migrate to anode but OH^- ions are discharged since they are lower in the electrochemical series.

Oxidation of the OH^- ions gives unstable hydroxyl radical which forms water with the liberation of oxygen.

Question 13

Give the electrode reactions for formation of

(i) Lead metal and bromine vapours from molten PbBr₂ using inert electrodes

(ii) H₂ and O₂ gas [2:1] from acidified water using inert Pt electrodes.

Answer

(i) Formation of Lead metal and bromine vapours from molten PbBr₂ using inert electrodes

At cathode:
Pb²⁺ + 2e^– ⟶ Pb

At anode:
2Br^– – 2e^– ⟶ Br
Br + Br ⟶ Br₂

(ii) Formation of H₂ and O₂ gas [2:1] from acidified water using inert Pt electrodes.

At cathode:
4H⁺ + 4e^– ⟶ 4H
2H + 2H ⟶ 2H₂

At anode:
4OH⁺ – 4e^– ⟶ 4OH
4OH ⟶ 2H₂O + O₂

Question 14

Starting from aq. copper (II) sulphate solution, give equations for the reactions at the cathode and anode during electrolysis of aq. CuSO₄ using active copper electrodes.

Answer

Dissociation of aq. CuSO₄ :

CuSO₄ ⇌ Cu²⁺ + SO₄^2-

H₂O ⇌ H¹⁺ + OH^1-

Electrode reaction:

Reaction at Cathode:
Cu²⁺ + 2e^- ⟶ Cu [Product : Copper metal]

Reaction at anode:
Cu – 2e^- ⟶ Cu²⁺ [Product : Nil , Cu²⁺ ions ]

Question 15

Give reasons for the following changes —

(i) pure water a non-electrolyte — becomes an electrolyte on addition of dil. H₂SO₄

(ii) Blue colour of aq. CuSO₄ — turns almost colourless on it's electrolysis using Pt electrodes.

Answer

(i) Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H₂SO₄ which dissociates as: H⁺¹ and SO₄^2- and help in dissociating water into H⁺¹ and OH^- ions, water being a polar solvent. Hence, pure water a non-electrolyte, becomes an electrolyte on addition of dil. H₂SO₄

(ii) The blue colour of CuSO₄ solution is due to the Cu²⁺ ions. During it's electrolysis using Pt electrodes, at the cathode Cu²⁺ ions are discharged as neutral copper atoms by accepting electrons. These are not replaced by Cu²⁺ ions from the Anode because at the Anode, OH^- ions are discharged. Hence, the blue colour of CuSO₄ solution fades and it becomes almost colourless on it's electrolysis using Pt electrodes.

Question 16

'Iron is electroplated with silver' —

(i) define the term in italics

(ii) state two reasons for electroplating

(ii) state why the iron is not placed at the anode and silver at the cathode during electroplating.

Answer

(i) Electroplating — It is the electrolytic process of deposition of a superior metal [i.e., nickle, silver, chromium, gold] on the surface of a baser metal or article [e.g., iron, copper, brass]

(ii) Reasons for electroplating:

1. Prevents corrosion or rusting — of the base metal.
2. Makes the article attractive and gives it an expensive appearance.

(iii) The article to be electroplated (i.e., iron) is always placed at the cathode because during electrolytic reaction the metal is always deposited at the cathode by gain of electrons.
The metal to be plated on the article (i.e., silver) is always made the anode because the metal anode continuously dissolves as ions in solution and is hence replaced periodically.

Question 17

Draw a diagram for —

(i) electroplating an article with silver;

(ii) electrorefining or purification of copper.

Answer

(i) Below diagram shows electroplating an article with silver:

(ii) Below diagram shows electrorefining or purification of copper:

Question 18

State the (i) electrolyte (ii) cathode used (iii) anode used (iv) electrode reaction at cathode (v) electrode reaction at anode (vi) product at cathode and anode

during

(a) electroplating an article with nickel (b) electroplating a spoon with silver (c) purification of impure copper.

Answer

(a) Electroplating an article with nickel :

1. Electrolyte — Aqueous solution of Nickle Sulphate
2. Cathode used — Cleaned article to be electroplated
3. Anode used — Active electrode. Plate or block of nickle metal
4. Electrode reaction at cathode — Ni²⁺ + 2e^- ⟶ Ni [deposited]
5. Electrode reaction at anode — Ni - 2e^- ⟶ Ni²⁺
6. Product at cathode — Nickel. (It is deposited on the article.)
   Product at anode — Nil (Ni²⁺ ions formed)

(b) Electroplating a spoon with silver :

1. Electrolyte — Sodium argentocyanide Na[Ag(CN)₂]
2. Cathode used — Cleaned article to be electroplated
3. Anode used — Active electrode. Plate or block of Silver metal
4. Electrode reaction at cathode — Ag¹⁺ + 1e^- ⟶ Ag [deposited]
5. Electrode reaction at anode — Ag - 1e^- ⟶ Ag¹⁺
6. Product at cathode — Silver. (It is deposited on the article.)
   Product at anode — Nil (Ag¹⁺ ions formed)

(c) Purification of impure copper :

1. Electrolyte — Aqueous copper sulphate solution [acidified]
2. Cathode used — Pure thin sheet of copper
3. Anode used — Impure block of copper
4. Electrode reaction at cathode — Cu²⁺ + 2e^- ⟶ Cu [deposited]
5. Electrode reaction at anode — Cu - 2e^- ⟶ Cu²⁺ [cation]
6. Product at cathode — Pure Copper. (It is deposited over thin sheet of pure Copper.)
   Product at anode — Nil (Cu²⁺ ions formed)

Question 19

Give a reason why the metals – copper, silver and lead are electrorefined but K, Na and Ca are not.

Answer

Method of extraction of metals depends on position of the metal in the activity series. Extraction of K, Na and Ca are done by electrolysis (Electrometallurgy). They are higher in electrochemical series and are extracted by electrolysis of their fused salts. On electrolysis metal is deposited at the cathode. Thus, they are already deposited at the cathode in pure state and hence need not be refined further.
On the other hand, lead and copper are extracted by using reducing agents and silver through thermal decomposition. Hence, they contain impurities and need to be generally electro-refined by electrolysis.

Question 20

Explain the term 'electrometallurgy'. At which electrode is the extracted metal always deposited ?

Answer

The process of extraction of metals by electrolysis is called electrometallurgy. Metals higher in the electrochemical series are extracted by electrolysis. During this process, the extracted metal is always deposited at the cathode.

Question 21

State how activity series of metals plays a role in extraction of metals from oxides.

Answer

Depending on the ease with which the metals lose their electrons and form ions they are arranged in metal activity series or electrochemical series.

The arrangement is so done that the elements that ionize most readily [discharged with great difficulty] are placed at the top of the series and other elements in the descending order.

Metals at the top of the series (eg. K, Na, Ca, Mg, Al) ionize most readily. Being highly reactive, they cannot be reduced by common reducing agent like C, CO, H₂.

Metals at the middle of the activity series (eg. Zn, Fe, Pb, Cu), being less reactive, can be extracted from their ores by reduction with common reducing agents like C, CO, H₂.

Metals near the bottom of the activity series (eg. Hg, Ag), due to their very low reactivity, can be extracted from their ores, by heating only.

Metal at the bottom of the activity series (Au, Pt) exist in native state .

Question 22

State the electrode reaction at the respective electrodes during extraction of Al from Al₂O₃.

Answer

Reaction:
Al₂O₃ ⇌ 2Al³⁺ + 3O^2-

Reaction at cathode:
2Al³⁺ + 6e^- ⟶ 2Al

Reaction at anode:
3O^2- – 6e^- ⟶ 3[O]
3[O] + 3[O] ⟶ 3O₂

Unit Test Paper 5 — Electrolysis

Question 1

Match the statements 1 to 5 with their answers selected from A to J.

A: Cathode, B: Sucrose soln., C: Cl^1-, D : Formic acid, E: Electrometallurgy, F: Ammonia, G: Mg²⁺, H: Electrorefining, I: Sulphur dioxide, J: Anode.

1. A compound containing molecules only.
2. A compound which ionizes in soln. state but not in gaseous state.
3. The ion which accepts electrons from the cathode and gets reduced to neutral atoms.
4. The electrode to which the cyanide ions of aq. Na[Ag(CN)₂] migrate during electrolysis.
5. An application of electrolysis in which the anode does not generally diminish in size.

Answer

The matched statements are given below:

Question 2

Complete the table given below:

Answer

The completed table is given below:

Question 3

Select the correct word from the words in bracket to complete the sentence:

1. The electrode at which anions donate excess electrons and are oxidized to neutral atoms is the ............... [anode/cathode]

2. On electrolysis, Ag¹⁺ and H¹⁺ ions migrate to the ............... [cathode/anode] and ............... [Ag¹⁺/H¹⁺] are discharged.

3. Electrolysis is a / an ............... [oxidation/reduction/redox] reaction in which reduction reaction takes place at the ............... [cathode/anode].

4. According to Arrhenius's theory the amount of electricity conducted by the electrolyte depends on the ............... [nature/concentration] of the ions in solution.

5. Salts ionize in aq. soln. on passage of electric current to give ............... [negative/positive] ions other than H⁺ ions.

Answer

1. The electrode at which anions donate excess electrons and are oxidized to neutral atoms is the anode

2. On electrolysis, Ag¹⁺ and H¹⁺ ions migrate to the cathode and Ag¹⁺ are discharged.

3. Electrolysis is a redox reaction in which reduction reaction takes place at the cathode .

4. According to Arrhenius's theory the amount of electricity conducted by the electrolyte depends on the concentration of the ions in solution.

5. Salts ionize in aq. soln. on passage of electric current to give positive ions other than H⁺ ions.

Question 4

Give balanced equations for the electrode reactions involved in the following conversions at the respective electrodes:

1. Aluminium oxide → Oxygen gas ← Copper [II] Sulphate
2. Copper metal → Copper ions → Copper metal
3. Lead [II] chloride → Chlorine gas ← Hydrochloric acid
4. Hydroxyl ions ← Acidified water → Oxygen gas
5. Potassium bromide → Bromine gas ← Lead bromide

Answer

1. Aluminium oxide → Oxygen gas ← Copper [II] Sulphate
   
   Electrolysis of molten Al₂O₃:
   Al₂O₃ ⇌ 2Al³⁺ + 3O^2-
   Reaction at anode:
   3O^2- - 6e^-⟶ 3[O]
   3[O] + 3[O] ⟶ 3O₂ [product O₂ gas]
   
   Electrolysis of copper (II) sulphate:
   CuSO₄ ⇌ Cu²⁺ + SO₄^2- [ions present]
   H₂O ⇌ H¹⁺ + OH^1-
   Reaction at anode:
   OH^1- - 1e^-⟶ OH x 4
   4OH ⟶ 2H₂O + O₂ [product oxygen gas]
2. Copper metal → Copper ions → Copper metal
   
   Reaction at anode:
   Cu – 2e^- ⟶ Cu²⁺
   Reaction at cathode:
   Cu²⁺ + 2e^- ⟶ Cu
3. Lead [II] chloride → Chlorine gas ← Hydrochloric acid
   
   Electrolysis of molten PbCl₂:
   PbCl₂ ⇌ Pb²⁺ + 2Cl^-
   Reaction at anode:
   2Cl^- – 2e^- ⟶ 2[Cl]
   Cl + Cl ⟶ Cl₂
   
   Electrolysis of hydrochloric acid:
   HCl ⇌ H⁺ + Cl^-
   Reaction at anode:
   2Cl^- – 2e^- ⟶ 2[Cl]
   Cl + Cl ⟶ Cl₂
4. Hydroxyl ions ← Acidified water → Oxygen gas
   
   Electrolysis of acidified water
   H₂SO₄ ⇌ 2H¹⁺ + SO₄^2-
   H₂O ⇌ H¹⁺ + OH^1-
   Reaction at anode:
   OH^1- - 1e^- ⟶ OH x 4
   4OH ⟶ 2H₂O + O₂ [product oxygen gas]
5. Potassium bromide → Bromine gas ← Lead bromide
   
   Electrolysis of molten KBr:
   KBr ⇌ K¹⁺ + Br^1-
   Reaction at anode:
   Br^- – 1e^- ⟶ Br
   Br + Br ⟶ Br₂
   
   Electrolysis of molten PbBr₂:
   PbBr₂ ⇌ Pb²⁺ + 2Br^-
   Reaction at anode:
   Br^- – 1e^- ⟶ Br
   Br + Br ⟶ Br₂
   
   

Question 5

Select the correct answer from the list in bracket:

1. The cation discharged at the cathode most readily.
   [Fe²⁺, Cu²⁺, Pb²⁺, H¹⁺]
2. The anion discharged at the anode with most difficulty.
   [SO₄^2-, Br^1-, NO₃^1-,OH^1-]
3. The metallic electrode which does not take part in an electrolytic reaction.
   [Cu, Ag, Fe, Ni]
4. The ion/s which is/are discharged during electrolysis of aq. CuSO₄ using Cu electrode.
   [Cu²⁺, SO₄^2-, H¹⁺, OH^1-]
5. A covalent compound which in aqueous state conducts electricity.
   [CCl₄, CS₂, NH₃, C₂H₄]

Answer

1. The cation discharged at the cathode most readily — Cu²⁺
2. The anion discharged at the anode with most difficulty — SO₄^2-
3. The metallic electrode which does not take part in an electrolytic reaction —Fe
4. The ion which is discharged during electrolysis of aq. CuSO₄ using Cu electrode — Cu²⁺
5. A covalent compound which in aqueous state conducts electricity — NH₃

Question 6.1

Give reason for the following :

Electrolysis of molten lead bromide is considered a redox reaction.

Answer

Electrolysis of molten lead bromide involves oxidation and reduction reaction and hence is a redox reaction. The reactions are as follows :

Reaction at the cathode:
Pb²⁺ + 2e^- ⟶ Pb [Reduction reaction]

Reaction at anode:
Br^1- - 1e^- ⟶ Br
Br + Br ⟶ Br₂ [Oxidation reaction]

Question 6.2

Give reason for the following :

Lead bromide undergoes electrolytic dissociation in the molten state but is a non-electrolyte in the solid state.

Answer

Solid lead bromide is a non-electrolyte since it's ions are not free but held together by an electrostatic force of attraction. These ions become free when lead bromide is in fused or molten state, hence it undergoes electrolytic dissociation in the fused or molten state.

Question 6.3

Give reason for the following :

The blue colour of aq. copper sulphate does not change when it is electrolysed using copper electrodes.

Answer

For every copper ion (Cu²⁺) discharged at the cathode as neutral copper atom (Cu), a copper ion (Cu²⁺) is added to the solution at the anode. Hence, total number of Cu²⁺ remains same. Therefore, the blue colour of CuSO₄ which is due to the presence of Cu²⁺ ions does not fade [SO₄^2- and OH^1- ions are spectator ions].

Question 6.4

Give reason for the following :

During electrolytic dissociation of sodium chloride, the sodium ions are discharged at the cathode.

Answer

Cathode is an electrode connected to the negative terminal of the battery, the electrode hence acquires a negative charge and positively charged sodium ions (Na⁺) migrate to negative electrode, i.e., cathode.

Question 6.5

Give reason for the following :

In the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid.

Answer

As nitric acid is a volatile acid, it may decompose and nitrate radical (NO₃^1-) may tend to interfere with the electrolytic reaction. Hence, in the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid.